KR20090110298A - A method of heat treatment for desensitizing a nickel-based alloy relative to environmentally-assisted cracking, in particular for a nuclear reactor fuel assembly and for a nuclear reactor, and a part made of said alloy and subjected to said treatment - Google Patents

Abstract

The invention relates to a processing method for cracking desensitisation using a nickel-based alloy environment having the following composition in wt %: C <= 0,10%; Mn <= 0,5%; Si <= 0,5%; P <= 0,015%; S <= 0,015%; Ni >= 40%; Cr = 12-40%; Co <= 10%; Al <= 5%; Mo = 0,1-15%; Ti <= 5%; B 0,01%; Cu 5%; W = 0,1-15%, Nb = 0-10%, Ta = 10%; the balance consisting of Fe, and unavoidable impurities resulting from the manufacturing, characterised in that said alloy is maintained at 950-1160°C in an atmosphere containing at least 100 ppm of hydrogen mixed with an inert gas or in pure hydrogen. The invention also relates to a part made of said Ni-based alloy having the above composition and subjected to said thermal processing.

Description

A heat treatment method for reducing the sensitivity of nickel-based alloys to environmental fuel cracking in a reactor fuel assembly and reactors, and components made of such alloys and heat treated by such heat treatment methods ALLOY RELATIVE TO ENVIRONMENTALLY-ASSISTED CRACKING, IN PARTICULAR FOR A NUCLEAR REACTOR FUEL ASSEMBLY AND FOR A NUCLEAR REACTOR, AND A PART MADE OF SAID ALLOY AND SUBJECTED TO SAID TREATMENT}

TECHNICAL FIELD The present invention relates to metallurgical techniques of nickel base alloys, and more particularly to alloys used to produce structural components of a reactor or structural components of a fuel assembly inserted into such a reactor.

To form a cooling circuit of components of the reactor, for example heat exchangers, cluster guide fins, pipe structures, reactors made of steel and having a heat transfer fluid in the form of gas, molten salt, liquid metal Fasteners and the like for fixing the components used are made of nickel-based alloys, for example, various types of Inconel®. At high temperatures and pressures, such components need to have excellent resistance to oxidation, corrosion, creep, periodically occurring thermal and mechanical stresses, and for such a long time (decades). There is a need, and nickel-based alloys also adapt well to that purpose.

The fuel assembly of a light reactor reactor may be made of a nickel-based alloy, with some of its structural components being a preferred example, 718 alloy. This applies in particular to grid springs typically made from strips of such alloys, flat semi-finished products for spring blades or hold-down springs made from wires for coil springs, and fastener elements made from bars. .

Nickel-based alloys that can be used in these situations have general components in a limited weight ratio as follows. C ≤ 0.10%, Mn ≤ 0.5%, Si ≤ 0.5%, P ≤ 0.015%, S ≤ 0.015%, Ni ≥ 40%, Cr = 12%-40%, Co ≤ 10%, Al ≤ 5%, Mo = 0.1% -15%, Ti 5%, B 0.01%, Cu 5%, W = 0% -15%, Nb = 0% -10%, Ta 10%. The rest are Fe and impurities which inevitably occur during the heat treatment process. These elements, for which no minimum is given, may not exist at all, or may only exist in trace amounts. Small amounts of other elements, which are used very rarely for the purpose of regulating certain chemical or mechanical properties, may be provided in small amounts and may be used in environmentally-assisted cracking, which results in stress corrosion in aqueous media. A small amount of other factors can be provided that do not dramatically alter the behavior of the alloy with regard to sensitivity.

Typically, the components of the 718 alloy, which are specific examples of such alloys, are as follows. C ≤ 0.08%, Mn ≤ 0.35%, Si ≤ 0.35%, P ≤ 0.015%, S ≤ 0.015%, Ni = 50%-55%, Cr = 17%-21%, Co ≤ 1%, Al ≤ 0.2% 0.8%, Mo = 2.8%-3.3%, Ti = 0.65%-1.15%, B <0.006%, Cu <0.3%, Nb + Ta = 4.75%-5.5%. The rest are Fe and impurities which inevitably occur during the heat treatment process. The alloy may also contain several hundred ppm of Mg.

An increasing importance in the operation of nuclear reactors containing such components is the ability of such components to withstand environmentally prone cracks. First, it is desirable to extend the duration of the operating cycle of the fuel assembly as much as possible. Thus, it is desirable to extend the current 12 month general duration to 18 months or even 24 months. Second, conditions specific to the primary medium in the LWR reactor are suitable for the occurrence of environmental cracking. This applies equally to reactors where gases, molten salts or liquid metals are used as heat transfer fluids when the oxidation phenomenon is exacerbated by reaching very high temperatures. In the case of pressurized water reactors, it has been empirically shown that grid springs, in particular made of alloy 718, may be fractured during use, as a result of the development of environmentally promoted cracking phenomena, in particular stress corrosion cracking (SCC). Fractures or cracks have also been found in cluster guide pins made of X750 alloy, pipe structures of steam generators made of 600 alloy, bushings of vessel bottoms, and weld zones. All of these parts are made of nickel-based alloys of various grades.

In order to improve the reliability of components made of nickel-based alloys, especially those made of 718 alloys, it is necessary to find a means to reduce the sensitivity to environmentally induced cracking of such components.

The solutions that have been used so far include, among other things, good industrial practice or mitigation measures.

Therefore, a method for changing the surface state of structural components mechanically (shot blasting, microbeading, sand blasting ...) or chemically (electro-polishing) has been proposed. For example, JP-A-2000 053 492 discloses a method of removing by oxidizing and electrochemically polishing the outermost surface layer of a single crystal casting material of a nickel base superalloy. Subsequently, the heat treatment is performed at a temperature equal to or higher than the recrystallization temperature. As a result, residual surface stresses in the material, which are sensitive to environmentally induced cracking phenomena, are eliminated. The surface is then covered with a ceramic layer. This publication discloses the application of this method to a gas turbine blade, but the method of changing the surface state of the material to remove residual stress has also been carried out for tubes of steam generators made of 600 and 690 alloys.

Another method is to apply a suitable coating to the material. Therefore, it is common to nickel plate grid springs made of alloy 718 to reduce the number of spring breaks in use. Other types of coatings, for example surface treatment by diffusion, are also possible. Thus, US-A-5 164 270 discloses a method of implanting Nb and / or Zr on the surface of an iron alloy containing 9% to 30% Cr and exposing it to a gas mixture of O ₂ and S. This also applies to nickel base alloys.

Another solution is to change the microstructure of the material by performing heat treatment at high temperatures (1100 ° C.) on the structural elements as a whole or locally. Therefore, local heat treatment is performed on the bend of the steam generator made of 600 alloy. In this way an attempt was also made to remove all traces of the δ state in the 718 alloy (see US-A-5 047 093).

Another solution is to change the chemical composition of the material in a very extreme or not extreme manner that can sometimes lead to the development of new alloy grades. Thus, in making the tubes of the steam generator, alloy 600 was replaced with alloy 690. Such methods are expensive to research and develop, and do not always lead to technically and / or economically viable results in terms of industrial availability.

Finally, no action is taken on the material itself in order to reduce the stress level the material is subjected to, but only on the design of the structure. Such a method is, in any case, expensive to develop, and often ends in failure.

In general, these rules for good practice lean toward optimizing the structure's ability to withstand the stresses that the material is subjected to, rather than permanently and clearly improving the properties of the material to access the essential features of the material. have.

It is an object of the present invention to improve the performance and reliability of the components of a reactor, made of nickel-based alloys, which, regardless of design, in particular in order to extend the duration of the operating cycle, are subjected to conditions which cause environmental cracking. To provide a means to do so. Such means must also be able to remove the material's sensitivity to environmentally responsible cracking phenomena with little or no conflict with other features of the material.

In order to achieve the above object, the present invention is a heat treatment method for reducing the sensitivity of nickel-based alloys to environmental cracking phenomenon, the alloy is a component of the weight ratio defined as follows, that is, C ≤ 0.10%, Mn ≤ 0.5%, Si ≤ 0.5%, P ≤ 0.015%, S ≤ 0.015%, Ni ≥ 40%, Cr = 12%-40%, Co ≤ 10%, Al ≤ 5%, Mo = 0.1%-15%, Ti ≤ 5%, In the heat treatment method of B ≤ 0.01%, Cu ≤ 5%, W = 0.1%-15%, Nb = 0%-10%, Ta ≤ 10%, the remainder is Fe and impurities inevitably generated during the heat treatment process And an alloy is maintained at a temperature of 950 ° C to 1160 ° C in an atmosphere of pure hydrogen or in an atmosphere containing at least 100 ppm hydrogen mixed with an inert gas.

Heat treatment to reduce the sensitivity to the environmental promoting cracking may be performed at a temperature range of 950 ℃ to 1010 ℃.

Heat treatment to reduce the sensitivity to the environmental promoting cracking can be carried out in a temperature range of 1010 ℃ to 1160 ℃.

The heat treatment to reduce the sensitivity to the environmental promoting cracking phenomenon can be performed on the semifinished product, which is subsequently subjected to a treatment for changing the metallurgical structure.

The heat treatment can be annealing, recrystallization, solid solution heat treatment, or curing, also referred to as aging.

A heat treatment to reduce the sensitivity to the environmentally responsible cracking phenomenon can be performed on the product, which is not subsequently subjected to a treatment to change its metallurgical structure.

After reducing the susceptibility to environmentally responsible cracking, the alloy can be machined or polished.

Heat treatment to reduce the sensitivity may be performed in the presence of a compound having a greater affinity for oxygen than the alloy.

The compound is a metal such as Al, Zr, Ti, Hf, an alloy containing at least one of these metals, an element such as Mg, Ca, or a compound of such an element.

Nickel-based alloys may be wrapped by sheets of the metal, alloy, or compound that have a greater affinity for oxygen than the nickel-based alloys, while at least a heat treatment is performed to reduce sensitivity to environmentally induced cracking phenomena.

The box having one or more walls made of the metal, alloy, or compound having a greater affinity for oxygen than the nickel-based alloy while at least a heat treatment is performed to reduce sensitivity to environmentally induced cracking phenomena. Can be disposed within.

The nickel-based alloy may be placed in the powder of the metal, alloy, or compound that has a greater affinity for oxygen than the nickel-based alloy while at least a heat treatment is performed to reduce sensitivity to environmentally induced cracking phenomena.

The alloy is composed of components with a defined weight ratio as follows: C <0.08%, Mn <0.35%, Si <0.35%, P <0.015%, S <0.015%, Ni = 50%-55%, Cr = 17%-21 %, Co ≤ 1%, Al ≤ 0.2%-0.8%, Mo = 2.8%-3.3%, Ti = 0.65%-1.15%, B ≤ 0.006%, Cu ≤ 0.3%, Nb + Ta = 4.75%-5.5% The remainder may be Fe and impurities which inevitably occur during the heat treatment process.

The invention also relates to components of a defined weight ratio as follows: C <0.10%, Mn <0.5%, Si <0.5%, P <0.015%, S <0.015%, Ni> 40%, Cr = 12%-40% , Co ≤ 10%, Al ≤ 5%, Mo = 0.1%-15%, Ti ≤ 5%, B ≤ 0.01%, Cu ≤ 5%, W = 0.1%-15%, Nb = 0%-10%, In the component manufacturing method of manufacturing a part with Ta ≤ 10%, the remainder is a Fe-based nickel-based alloy which is an inevitable impurity during the heat treatment process, to reduce the sensitivity of the alloy to the above-mentioned environmental promoting cracking phenomenon It provides a component manufacturing method comprising a heat treatment.

The present invention also provides a component made of a nickel-based alloy, wherein the alloy has been subjected to a heat treatment to reduce the sensitivity of the alloy to the above-mentioned environmental promoting cracking phenomena.

The component may be a structural element of the reactor fuel assembly.

The component can be a grid spring, a hold-down assembly, or a screw.

The parts are composed of components with a defined weight ratio as follows: C <0.08%, Mn <0.35%, Si <0.35%, P <0.015%, S <0.015%, Ni = 50%-55%, Cr = 17%- 21%, Co ≤ 1%, Al ≤ 0.2%-0.8%, Mo = 2.8%-3.3%, Ti = 0.65%-1.15%, B ≤ 0.006%, Cu ≤ 0.3%, Nb + Ta = 4.75%-5.5 % And the remainder may be made of Fe and nickel-based alloys which are inevitable impurities during the heat treatment process.

The component may be an element of the cooling circuit of the reactor.

The component may be a pipe, cluster guide pin, spring, heat exchanger, screw, bolt, or other component made of a nickel base alloy and in contact with the heat transfer fluid.

The part can be a semifinished product which can be made into the part by shaping, machining or cutting methods.

The part may constitute a sheet, strip, wire, bar, or blank.

As can be seen from the above description, the present invention is first and foremost based on developing a heat treatment of the material to be performed in the presence of a powerful reducing agent, in a hydrogen atmosphere or in an atmosphere containing hydrogen. Through heat treatment, the alloy is permanently reduced in sensitivity to environmentally induced cracking phenomena by the mechanism described below.

Heat treatments to reduce this sensitivity can be used in place of, but in place of, those generally applied by those skilled in the art to obtain the expected mechanical characteristics.

The test piece taken from a strip made of 718 alloy was subjected to an annealing for 100 hours (h) at an temperature of 980 ° C. in an Ar-H ₂ (5%) gas mixture, followed by heat treatment. It was found to have a significantly lower sensitivity to brittle fractures due to coarse cracking, which was found to be further removed after polishing the outer surface of the specimen.

This observation gave the inventors a clue to the modification of the components of the 718 alloy and similar materials by reducing the content of carbon, oxygen and nitrogen at least near the surface of the part. Therefore, the sensitivity of the component to environmental cracking can be greatly reduced, and the sensitivity of the component to intergranular cracking at high temperatures (> 350 ° C.) can be significantly reduced. This is a very suitable condition to be a structural element of the fuel assembly or the cooling circuit, which needs to be operated under conditions that may be problematic. This applies especially to pressurized water reactors (PWRs). However, the present invention can also be applied to boiling water reactors (BWRs), and also to reactors cooled by gas, molten salt or liquid metal, and intermediate temperatures (200 ° C.-500 ° C.) in liquid or gaseous media. Or other equipment that uses structural elements of nickel-based alloys that operate at high temperatures (500 ° C-1200 ° C) oxidation conditions.

Nevertheless, if a heat treatment to reduce the sensitivity is carried out on a less applicable microstructure, then the heat treatment to reduce the sensitivity is intended to restore the structural and mechanical properties of the alloy to adapt well to the intended application. It must be combined with other heat treatments and / or thermomechanical treatments.

The most likely mechanism for accounting for cracks in nickel-based alloys due to environmentally promoted cracking phenomena in the primary fluid of an aqueous medium, such as a hard water reactor, is as follows. This is based on the interparticle diffusion of oxygen atoms derived from the dissociation of water constituting the primary fluid. Various mechanisms can occur at grain boundaries that degrade mechanical strength. In particular, the following mechanism occurs.

The formation of CO and CO ₂ by oxidation of carbon,

The formation of one or more embrittling oxides, such as Cr ₂ O ₃ ,

Intrinsic embrittlement at the particle boundary by oxygen, and

The release of sulfur which is very embrittled by the reaction of precipitates and oxygen, which are sulfur-containing impurities, which arise during the heat treatment process.

Similar mechanisms exist with other heat transfer fluids. In such a situation, oxygen atoms come from the medium itself or from the impurities present in the surrounding medium, and even smaller amounts of oxygen are compensated by the high operating temperatures of the components made of nickel base alloys.

Conventional research (article titled "Oxidation resistance and critical sulfur content of single-crystal superalloys" by JL Smialek, international gas turbine and aviation engine According to the International Gas Turbine and Aeroengine Congress & Exhibition, Birmingham, June 10-13, 1996, prolonged exposure to high temperatures (1200 ° C-1300 ° C) in a hydrogen-containing atmosphere (8 hours) 100 hours) showed that sulfur was removed from the surface of the single crystal nickel-based alloy by evaporation of H ₂ S. This method is intended to reduce the problem of material fracture. Nevertheless, this method can be applied to nickel-based superalloys that are not single crystals. In such a situation, high temperatures cause the growth of the particles and cause deformation of the crystal structure, which is not necessarily desirable.

Thus, the inventors conducted a primary test on test pieces taken from strips having the following components. C ≤ 0.016%, Ni = 53.7%, B = 0.0009%, Mn = 0.11%, Mg = 0.0087%, Mo = 2.88%, Fe = 18.03%, Si = 0.12%, Al = 0.54%, Co = 0.04%, P = 0.005%, Cu = 0.03%, S = 0.00034%, Ti = 1.04%, Cr = 18.1%, Nb + Ta = 5.15%. Treatment was performed using NiCoCrAlYTa powder covering the sample while the Ar-H ₂ (5%) mixture was flowing to reduce the spatial pressure of oxygen. Subsequently the following steps were performed.

Heat treatment was performed at a temperature of 980 ° C. for 100 hours. This heat treatment limits particle growth, but when it is desirable to avoid environmentally promoting cracking phenomena, a δ state precipitation, which is generally considered undesirable, is caused.

The growth of particles was also caused by keeping the temperature at 1080 ° C. for 1 hour to return the δ state to the resolved state.

Curing (aging) at a temperature of 720 ° C. for 8 hours or curing (aging) at a temperature of 620 ° C. for 8 hours.

After the treatment, the reactor smelled of H ₂ S. Nevertheless, minute analysis by light emission mass spectroscopy did not show a significant drop in the amount of sulfur, but a very significant drop in the amount of carbon, nitrogen and, inter alia, oxygen.

The traction test, applied at a rate of 10 ⁻³ s ⁻¹ at a temperature of 650 ° C. in air, resulted in specimen failure with some initial intergranular cracks, but the amount was significantly smaller than that of the unannealed reference sample.

Intraparticle soft fracture surfaces were obtained as a whole by polishing each face of the same specimen as the previous one over 15 μm to remove surface areas that did not reduce overall sensitivity.

Polishing is an optional operation. Incorporating polishing into a process that reduces sensitivity can reduce the duration of heat treatment.

In contrast, the test piece heat-treated and polished under the above conditions continued to exhibit intergranular wavefront, except when H ₂ was not present in the heat treatment atmosphere.

The benefits of the treatment usually stem from the nature of the atmosphere, which is greatly reduced during the heat treatment.

Oxygen, carbon, and nitrogen present in the alloy are removed, in particular from the grain boundaries.

-Prevent the surface of the sample from oxidizing;

 This removal of brittleness at the grain boundaries is advantageous for reducing the material's sensitivity to environmentally induced cracking phenomena.

Subsequently, tests were conducted to confirm the good results described above and to determine the appropriate range of heat treatment.

As a sample, a sheet of 0.27 mm thickness was used, which was known to exhibit high sensitivity to environmentally induced cracking (cracking observed when used in a reactor).

In order to avoid austenite grain growth and to limit the δ state precipitation, the heat treatment temperature to reduce the sensitivity was set to 990 ° C ± 10 ° C.

The treatment atmosphere was Ar-H ₂ (5%).

The sample was made of a FeCrAlY alloy with the following components: Al = 5%, C = 0.02%, Cr = 22%, Mn = 0.2%, Si = 0.3%, Y = 0.1%, Zr = 0.01%, and the balance = Fe I wrapped it with a sheet.

The duration of the heat treatment to reduce the sensitivity was up to 100 hours.

The quality of the decrease in sensitivity to environmental cracking was evaluated as follows.

Traction tests were performed in air at 650 ° C. at a rate of about 10 ⁻³ s ⁻¹ . The results associated with the failure mode were considered to be representative of those obtained under high temperature conditions in a gas medium, molten salt medium, or liquid metal medium.

PWR primary medium (de-aired pure water with a pH equal to 6.4 at 25 ° C. and containing 1200 ppm boron added in the form of lithium and boric acid added in the form of lithine) A slow traction test was performed at a temperature of 350 ° C. (at a speed of approximately 1.7 × 10 ⁻⁸ s ⁻¹ ). F ^-, Cl ^-, and the partial pressure of hydrogen is less than if So ₄ ^-2 30 ppb amounts of 0.5 was immediately set, to simulation to approximate as possible to the most sensitive areas of the image of the grid to the support legs environment promotes cracking phenomenon Slow traction tests were performed on the V-shaped specimens.

Slow compression tests were performed on the grid springs after the process of reducing sensitivity was performed.

Specimens of 718 alloy had a cross-section of 2 x 0.27 mm ² or 3 x 0.27 mm ² and were provided with the following components. C = 0.016%, Ni = 53.7%, B = 0.0009%, Mn = 0.11%, Mg = 0.0087%, Mo = 2.88%, Fe = 18.03%, Si = 0.12%, Al = 0.54%, Co = 0.04%, P = 0.005%, Cu = 0.03%, S = 0.00034%, Ti = 1.04%, Cr = 18.1%, Nb + Ta = 5.15%.

The test pieces were kept in the atmosphere of Ar-H ₂ (5%) at a temperature of 980 ° C. for 30 minutes to 100 hours, and subsequently in the same atmosphere, according to the aging treatment normally applied to the relevant items. Alternatively, a heat treatment was performed to reduce the sensitivity to environmental crack cracking by aging at a temperature of 720 ° C. for 8 hours in a vacuum state and then 620 ° C. for 8 hours. In both baseline tests, no process was performed to reduce the sensitivity at 980 ° C. For one test, the step of covering the sample with a sheet of FeCrAlY was replaced by placing the specimen in a box made of FeCrAlY.

After the heat treatment was performed, the wavefront was examined to determine whether the wavefront is present in the intergranular (IG), in the particle (TG), or in both cases.

The results are summarized in Table 1.

Table 1: Heat Treatment Conditions for Towing Testing Samples at Temperatures of 650 ° C. in Air and 350 ° C. in PWR Primary Media—Test Results

exam Heat treatment atmosphere Type of destruction One 720 ℃ / 8h + 620 ℃ / 8h vacuum both side 2 720 ℃ / 8h + 620 ℃ / 8h Subsequent Polishing vacuum both side 3 980 ℃ / 100h + 1080 ℃ / 1h + 720 ℃ / 8h + 620 ℃ / 8h Ar-H ₂ / vacuum TG 4 980 ℃ / 96h + 720 ℃ / 8h + 620 ℃ / 8h Ar-H ₂ TG 5 980 ℃ / 48h + 720 ℃ / 8h + 620 ℃ / 8h Ar-H ₂ TG 6 980 ℃ / 48h + 720 ℃ / 8h + 620 ℃ / 8h FeCrAlY Box Ar-H ₂ TG 7 980 ℃ / 48h + 720 ℃ / 8h + 620 ℃ / 8h Ar-H ₂ / vacuum TG 8 980 ℃ / 39h + 720 ℃ / 8h + 620 ℃ / 8h Ar-H ₂ / vacuum TG or both 9 980 ℃ / 36h + 720 ℃ / 8h + 620 ℃ / 8h Ar-H ₂ TG or both 10 980 ℃ / 33h + 720 ℃ / 8h + 620 ℃ / 8h Ar-H ₂ / vacuum both side 11 980 ℃ / 30h + 720 ℃ / 8h + 620 ℃ / 8h Ar-H ₂ both side 12 980 ℃ / 27h + 720 ℃ / 8h + 620 ℃ / 8h Ar-H ₂ both side 13 990 ℃ / 24h + 720 ℃ / 8h + 620 ℃ / 8h Ar-H ₂ both side 14 980 ℃ / 240h + 720 ℃ / 8h + 620 ℃ / 8h Ar-H ₂ both side 15 980 ℃ / 21h + 720 ℃ / 8h + 620 ℃ / 8h Ar-H ₂ / vacuum both side 16 980 ℃ / 18h + 720 ℃ / 8h + 620 ℃ / 8h Ar-H ₂ / vacuum both side 17 980 ℃ / 15h + 720 ℃ / 8h + 620 ℃ / 8h Ar-H ₂ / vacuum both side 18 980 ℃ / 12h + 720 ℃ / 8h + 620 ℃ / 8h Ar-H ₂ both side 19 980 ℃ / 9h + 720 ℃ / 8h + 620 ℃ / 8h Ar-H ₂ / vacuum both side 20 980 ℃ / 6h + 720 ℃ / 8h + 620 ℃ / 8h Ar-H ₂ / vacuum both side 21 980 ℃ / 3h + 720 ℃ / 8h + 620 ℃ / 8h Ar-H ₂ / vacuum both side 22 980 ℃ / 1h + 720 ℃ / 8h + 620 ℃ / 8h Ar-H ₂ / vacuum both side 23 980 ℃ / 0h30 + 720 ℃ / 8h + 620 ℃ / 8h Ar-H ₂ / vacuum both side

The failure pattern was the same in both test conditions.

Specimens 1 and 2 were not subjected to heat treatment to reduce the sensitivity, and the wavefront exhibited intergranular brittleness and intragranular ductility.

Test pieces 3 to 23 were subjected to such heat treatment, and exhibited the following characteristics.

The wavefront showed intergranular brittleness and intragranular ductility. or

-The wavefront showed pure intragranular ductility characteristics.

The intergranular ductility of the wavefront was more pronounced when the heat treatment to reduce the sensitivity was performed for a long time. When the heat treatment time was 36 hours, some pure intraparticle wavefronts were found. When the heat treatment time exceeded 39 hours, all of the wavefronts were pure intraparticle wavefronts. Thus, the heat treatment duration in the range of 36 hours to 39 hours is the limit time of the heat treatment to reduce the sensitivity of the entire sample, and in such a situation, the reduction of the sensitivity, in part or in whole, depends on the variation of the heat treatment conditions such as temperature. do.

Thus, a heat treatment to reduce the sensitivity for at least 40 hours at a temperature of 980 ° C. is complete for these sheets to always obtain a complete reduction in the sensitivity of the material to environmental promoting cracking phenomena at a temperature of 650 ° C. It is effective to do.

The effect of heat treatment to reduce the sensitivity to the microstructure of the material can be explained as follows.

When the 718 alloy is heat treated at a temperature of 850 ° C to 1010 ° C, the δ state is precipitated in an amount depending on the temperature and the heat treatment time. The heating rate also has a great influence on the amount of δ state present, especially at high temperatures above 950 ° C. When the heating rate is slow, the δ state can be formed while the heating is performed. Therefore, depending on the holding temperature, the volume ratio of the δ state tends to increase when the temperature is low, or decreases and then stabilizes when the temperature lies in the upper portion of the acceptable range.

If the temperature exceeds approximately 1010 ° C. (the Solvus temperature in δ state, which can change little by little as a function of the exact composition of the alloy), the grain growth is exposed to consideration, so that microstructures are preferred for the present invention. Not very well adapted to the application.

In contrast, in the range of 980 ° C. to 1000 ° C., given sufficient retention time and for all possible components of the 718 alloy, small intergranular particles in the δ state can be removed, and insoluble precipitates will be spheroidized. Can be.

It is also important to note that by conducting comparative tests on samples subjected to heat treatment for 96 hours at Ar-H ₂ (5%) atmosphere or at a temperature of 980 ° C. under vacuum, the heat treatment atmosphere is also very important for heat treatment to successfully reduce sensitivity. Verified. It was clearly found that the sample treated in the vacuum state was broken during the traction test to have intergranular brittle wavefront, while the sample treated in the hydrogen containing atmosphere had an intragranular soft wavefront. Thus, the presence of a hydrogen-containing atmosphere containing at least 100 ppm of H ₂ and an inert gas such as Ar or consisting of a pure H ₂ is mixed to the chamber is indispensable in the present invention.

In the case of aging treatments which are carried out subsequent to treatments for reducing sensitivity in certain preferred applications of the invention, such as grid springs for fuel assemblies, it is generally desirable to carry out such treatments at temperatures not exceeding 760 ° C. . If the temperature is exceeded, the δ state precipitation is observed in the form of a film or fillet at the grain boundaries that combine with the densities of γ 'and γ "precipitates at the same location. Thus, an autoclave representing the primary conditions of PWRs. During testing within (350 ° C.), cracks are often found in samples that are stressed greater than or equal to the elastic limit of the alloy, and according to the conventional idea, excessively formed δ states at relatively low temperatures reduce sensitivity. It is more damaging to the sensitivity to environmental cracking phenomena than the δ state formed at relatively high temperatures (temperatures above 950 ° C) during the treatment.

The experiments performed by the inventors were carried out when the treatment to reduce the sensitivity to environmental cracking phenomena prior to aging (heat treatment at a temperature of 740 ° C. to 780 ° C. for 8 hours, then cooling in an oven) (980 ° C., 40 hours), in any case the sensitivity to environmental cracking has been removed, and thus aging carried out within its treatment range shows no detrimental effect on environmental cracking. Aging is simply a routine function of controlling the mechanical properties of the material. In this embodiment, aging increases the elastic limit.

An absolutely necessary condition for reducing the sensitivity of the alloy is that the heat treatment atmosphere is not oxidized, and more preferably, the atmosphere reduces the oxide layer generally innate on the surface of the material. Unless a pure hydrogen atmosphere is used, it is most desirable to carry out a treatment to reduce the sensitivity in the presence of a composite that captures oxygen having a greater affinity than the component being treated.

In order to achieve this object, a metal having a high oxidation affinity, such as Al, Ti, Hf, Zr, or at least one of a large amount of a metal, an element such as Mg, Ca, or an alloy containing a compound of such an element Or some other compound.

It is possible to cover the surface of the part with the powder of the above alloy, but there is a risk that the surface of the part will be sintered and contaminated by the powder, especially if the processing time is longer, and it becomes difficult to recover the part. Nevertheless, this method has been successfully tested in the current development situation.

Therefore, it is desirable to use two other techniques that have been found to be effective and that do not pose the risks associated with using powders.

The first technique is to wrap the part with a sheet of metal or alloy that acts as an oxygen trap.

The second technique is to place the part in a box with one or more walls made of the metal or alloy.

As a preferred but not exclusive example of such an alloy, there is a FeCrAlY alloy used during the test to reduce the above sensitivity. Used as components of catalytic converters in the automotive industry or as components of components for machine tools or electrical resistors, these materials are commonly available on the market and have been found to be very effective.

A test was also conducted to measure the sensitivity to environmentally induced cracking phenomena of grid springs made of alloy 718 having the same components as the traction specimens described above. These grid springs were tested at a temperature of 350 ° C. in a PWR primary medium. At this time, the displacement velocity was 10 ⁻⁷ s ^−1, and the displacement imposed during the test was in conformity with the design.

In the case of springs with only aging treatments without any pretreatment to reduce the sensitivity to environmentally induced cracking, many early cracks were found in three of the four legs of the spring, with intergranular fracture. It became.

As long as the initial crack associated with intergranular fracture was observed in only one leg and not much compared with no treatment, the sensitivity for 30 hours at 990 ° C in an Ar-H ₂ 5% atmosphere before aging was observed. Enhancements are made by processing to reduce them. However, reducing the sensitivity to environmental cracking is not complete.

In contrast, the springs treated to reduce sensitivity for 42 hours at a temperature of 990 ° C. showed no initial intergranular cracking. Thus, the spring was completely removed from susceptibility to environmentally induced cracking phenomena, which supported the experimental results obtained for the test specimens.

Other tests were performed on specimens of 718 alloys with components very similar to those described above, but in this case, due to the quantitative differences in interstitial elements (C, N, and O) present in various strip bundles, It is empirically shown that it is less susceptible to environmentally induced cracking phenomena prior to treatment to reduce the sensitivity than the specimens of.

In some situations, it has been found that an overall reduction in sensitivity to environmentally responsible cracking phenomena can be obtained after treatment for 15 hours at a temperature of 990 ° C ± 10 ° C. A decrease in sensitivity that is very significant but not always global could always be obtained after treatment for 30 hours at a temperature of 990 ° C. ± 10 ° C. When the treatment time exceeded 40 hours, a total reduction in sensitivity to environmental cracking phenomena was always obtained in air at 650 ° C. and in PWR primary media at 350 ° C.

Under such conditions, the following components: C ≦ 0.10%, Mn ≦ 0.5%, Si ≦ 0.5%, P ≦ 0.015%, S ≦ 0.015%, Ni ≧ 40%, Cr = 12% -40%, Co ≦ 10 %, Al ≤ 5%, Mo = 0.1%-15%, Ti ≤ 5%, B ≤ 0.01%, Cu ≤ 5%, W = 0.1%-15%, Nb = 0%-10%, Ta ≤ 10% With respect to the sensitivity to the environmental cracking phenomenon of the nickel-based alloy, which is an inevitably occurring impurity during Fe and the heat treatment process, the following treatment conditions according to the present invention are proposed. The 718 alloy constitutes an example that is preferred but not exclusive, and the heat treatment is performed as follows.

The atmosphere is composed of pure hydrogen or an inert gas, such as argon, in which at least 100 ppm of hydrogen is mixed. In the vicinity of the component to be treated, it is preferable to ensure the absence of oxygen by the presence of a compound having a greater affinity for oxygen than the nickel-based alloy.

The above compounds may be metals such as Al, Zr, Ti, Hf, alloys such as FeCrAlY alloys containing at least one of these metals, elements such as Mg, Ca, or compounds of such elements.

While at least a treatment for reducing sensitivity to environmental cracking is performed, the nickel-based alloy may be wrapped in a sheet of the compound having a greater affinity for oxygen, carbon, and nitrogen than the nickel-based alloy.

While at least a treatment to reduce the susceptibility to environmentally induced cracking is performed, the nickel-based alloy may be placed in a box having one or more walls made of the compound having a greater affinity for oxygen than the nickel-based alloy.

Nickel-based alloys may be embedded in the powder of the compound having a greater affinity for oxygen than the nickel-based alloys, while at least a treatment to reduce the sensitivity to environmentally induced cracking phenomena is performed.

The exact conditions for the minimum duration and the treatment temperature depend on the shape of the product or semifinished product whose sensitivity is to be reduced and also on the expected quality of the sensitivity reduction.

The temperature of the heat treatment to reduce the sensitivity may be in the temperature range of 950 ℃ to 1160 ℃. In general, one of the following two ranges is selected. 950 ° C-1010 ° C or 1010 ° C-1160 ° C.

The duration of the heat treatment to reduce the sensitivity can be determined using empirical formulas deduced from the experiment. For example, for sheets that are 0.3 mm thick and heat treated to temperatures between 980 ° C. and 1000 ° C., the following formula can be used to determine the minimum duration of treatment required to obtain a product with reduced overall sensitivity. have.

If initial brittleness (B) lies in the range of 0% to 10%, t (time) = 3.4 × (B%)

If initial brittleness (B) lies in the range of 10% to 50%, t (time) = 0.2 x (B%)

Here, the brittleness (B) of the material is defined as the ratio of the total length of the divided particle boundary fracture zone to the total length around the wavefront while the test is carried out in a medium indicative of the operating conditions of the component.

The selection of the treatment temperature range (950 ° C-1010 ° C or 1010 ° C-1160 ° C) depends essentially on the process step of the material on which the treatment is carried out and the requirements of the microstructure at the final stage of the treatment. Depends on.

In the semi-finished step, it is preferable to carry out the treatment at a higher temperature, and subsequent treatment is carried out in a process for regenerating the microstructure of the material if it is undesirably affected by a decrease in sensitivity.

It is preferable to carry out the treatment at a lower temperature in the finished product stage, and thus treatment at such a low temperature constitutes the final stage of the process. In addition, the size of the particles is generally not significantly affected by the treatment to reduce the sensitivity.

Nevertheless, this choice is not limited. The high temperature treatment can be performed on the finished product when there is no requirement imposed on the microstructure, for example as applied to cluster guide pins. Similarly, low temperature treatment can be performed on semi-finished products. Treatments with longer durations than at high temperatures are needed to achieve a reduction in overall sensitivity while keeping others the same.

Nevertheless, it may be desirable to reduce the duration of heat treatment, especially when carried out in the semifinished product stage. The resulting semifinished product, due to the edge effect leading to intensive stabilization of the elements at the interface between the metal and the treatment atmosphere, makes its surface still slightly sensitive to environmentally induced cracking at the end of the treatment. In such a situation, in order to obtain a product with reduced overall sensitivity, the heat treatment is finished by the operation of removing the surface layer which is not reduced overall.

The surface layer can be removed by machining and / or chemical, electrochemical and mechanical polishing.

If necessary, subsequent to heat treatment to reduce the susceptibility to environmentally induced cracking of the nickel-based alloy, the mechanical properties and microstructures necessary to accelerate the subsequent manufacturing process and ensure that the component behaves well during use. Annealing, recrystallization, solid solution heat treatment, or curing (also referred to as aging), as commonly applied by those skilled in the art, when processing semi-finished and finished products made of nickel-based alloys to ensure completeness with structure. Heat treatment) can be performed. One condition which is absolutely necessary is that these heat treatments must be carried out in a non-oxidizing atmosphere in order to avoid the material becoming sensitive again to environmentally prone cracking phenomena.

According to the invention, it is possible to obtain parts and semi-finished products as given in the incomplete list below.

The component thus produced may be a structural component of the fuel assembly for the reactor.

The component can be a grid spring, a hold-down assembly, or a screw.

The component may be an element of a reactor cooling circuit.

The component may be a pipe, cluster guide pin, spring, heat exchanger, screw, bolt, or other component made of a nickel base alloy and in contact with the heat transfer fluid.

The semifinished product can be, for example, a sheet, strip, wire, bar, or blank obtained by forging, stamping, casting, or sintering, and is produced from the semifinished product into parts by various conventional shaping, machining, or cutting methods. Can be.

The alloy 718 thus treated is particularly suited to the manufacture of grid springs and containment assembly spring components for reactor fuel assemblies. However, the alloy may also be used such that other components can be used in a manner compatible with mechanical properties and in the environment that are suitable for the occurrence of environmentally prone cracking during use.

Claims (22)

A heat treatment method for reducing the susceptibility of a nickel-based alloy to environmentally induced cracking, wherein the alloy has a limited weight ratio of components such as C ≤ 0.10%, Mn ≤ 0.5%, Si ≤ 0.5%, P ≤ 0.015%, S ≤ 0.015%, Ni ≥ 40%, Cr = 12%-40%, Co ≤ 10%, Al ≤ 5%, Mo = 0.1%-15%, Ti ≤ 5%, B ≤ 0.01%, Cu ≤ 5%, In the heat treatment method having W = 0.1%-15%, Nb = 0%-10%, Ta ≤ 10%, and the remainder are Fe and impurities which inevitably occur during the heat treatment process, the alloy is in the atmosphere of pure hydrogen or inert And at a temperature of 950 ° C. to 1160 ° C. in an atmosphere containing at least 100 ppm of hydrogen mixed with the gas. The method of claim 1, Heat treatment to reduce the sensitivity to the environmental promoting cracking phenomenon is carried out in a temperature range of 950 ℃ to 1010 ℃. The method of claim 1, Heat treatment to reduce the sensitivity to the environmental promoting cracking phenomenon is carried out in a temperature range of 1010 ℃ to 1160 ℃. The method according to any one of claims 1 to 3, And a heat treatment for reducing the sensitivity to the environmentally induced cracking phenomenon is performed on the semi-finished product, which is subsequently subjected to a treatment for changing the metallurgical structure. The method of claim 4, wherein The heat treatment is an annealing, recrystallization, solid solution heat treatment, or curing carried out in a non-oxidizing atmosphere. The method according to any one of claims 1 to 3, A heat treatment for reducing the sensitivity to environmentally induced cracking phenomena is performed on a product in which no treatment for changing the metallurgical structure is subsequently performed. The method according to any one of claims 1 to 6, After reducing the susceptibility to environmentally induced cracking, the alloy is machined or polished. The method according to any one of claims 1 to 7, Heat treatment to reduce the sensitivity is carried out in the presence of a compound having a greater affinity for oxygen than the alloy. The method of claim 8, And said compound is a metal such as Al, Zr, Ti, Hf, an alloy containing at least one of these metals, an element such as Mg, Ca, or a compound of such element. The method of claim 9, The nickel-based alloy is wrapped by a sheet of the metal, alloy, or compound that has a greater affinity for oxygen than the nickel-based alloy while at least a heat treatment is performed to reduce the sensitivity to environmentally induced cracking phenomena. Heat treatment method. The method of claim 9, The box having one or more walls made of the metal, alloy, or compound having a greater affinity for oxygen than the nickel-based alloy while at least a heat treatment is performed to reduce sensitivity to environmentally induced cracking phenomena. The heat treatment method characterized in that disposed within. The method of claim 9, The nickel-based alloy is placed in a powder of the metal, alloy, or compound having at least affinity for oxygen than the nickel-based alloy while at least a heat treatment is performed to reduce the sensitivity to environmentally induced cracking phenomena. Heat treatment method. The method according to any one of claims 1 to 12, The alloy is composed of components with a defined weight ratio as follows: C <0.08%, Mn <0.35%, Si <0.35%, P <0.015%, S <0.015%, Ni = 50%-55%, Cr = 17%-21 %, Co ≤ 1%, Al ≤ 0.2%-0.8%, Mo = 2.8%-3.3%, Ti = 0.65%-1.15%, B ≤ 0.006%, Cu ≤ 0.3%, Nb + Ta = 4.75%-5.5% And the remainder are Fe and impurities which are inevitably generated during the heat treatment process. Components of a defined weight ratio as follows: C <0.10%, Mn <0.5%, Si <0.5%, P <0.015%, S <0.015%, Ni> 40%, Cr = 12%-40%, Co <10 %, Al ≤ 5%, Mo = 0.1%-15%, Ti ≤ 5%, B ≤ 0.01%, Cu ≤ 5%, W = 0.1%-15%, Nb = 0%-10%, Ta ≤ 10% In the component manufacturing method for manufacturing a component from the Fe-based and nickel-based alloy which is an inevitable impurity during the heat treatment process, the alloy for the environmental cracking phenomenon according to any one of claims 1 to 12. Component manufacturing method comprising a heat treatment for reducing the sensitivity of the. A component made of a nickel-based alloy, wherein the alloy is subjected to a heat treatment to reduce the sensitivity of the alloy to environmentally induced cracking phenomena according to any one of claims 1 to 13. The method of claim 15, Said component being a structural element of a reactor fuel assembly. The method of claim 16, And said component is a grid spring, a hold-down assembly, or a screw. The method according to any one of claims 15 to 17, The parts are composed of components with a defined weight ratio as follows: C <0.08%, Mn <0.35%, Si <0.35%, P <0.015%, S <0.015%, Ni = 50%-55%, Cr = 17%- 21%, Co ≤ 1%, Al ≤ 0.2%-0.8%, Mo = 2.8%-3.3%, Ti = 0.65%-1.15%, B ≤ 0.006%, Cu ≤ 0.3%, Nb + Ta = 4.75%-5.5 And a remainder of which is made of a nickel-based alloy, which is Fe and an impurity inevitably generated during the heat treatment process. The method of claim 15, Said component being an element of a cooling circuit of said reactor. The method of claim 19, The component is a pipe, cluster guide pin, spring, heat exchanger, screw, bolt, or other component made of a nickel-based alloy and in contact with the heat transfer fluid. The method of claim 15, The part is a semi-finished product which can be made into the part by shaping, machining or cutting methods. The method of claim 21, The part comprises a sheet, strip, wire, bar, or blank.